Thermal solar concentration technology allows, as a result of the high temperatures obtained when concentrating solar radiation by means of an array of reflectors distributed throughout a solar field, obtaining hot air or steam which will be used to produce electricity following conventional processes.
Out of the different variants for generating electricity by concentrating solar radiation, the reflective element of the present invention is particularly advantageous for tower concentration technology.
Tower technology uses an array of mobile reflectors, also known as heliostats which, by means of the movement generally about two shafts, are capable of reflecting and concentrating solar radiation on a single central receiver located in the upper area of a tower.
Each heliostat in turn comprises a reflective sheet or surface which the radiation directly strikes, a bearing structure, the mechanisms so that the bearing structure performs the orientation movements for being oriented towards the solar radiation and attachment means between the reflective sheet and the bearing structure.
The reflective sheet can have a variable area, having a larger size in those parts of up to 120 m2. This sheet is actually made up of an array of mirrors, each being mounted on an underframe serving as an attachment element for attaching it to the structure. In a particular example, the reflective sheet is made up of 28 mirrors. The dimensions of each of these mirrors may vary among installations, but an example of a mirror size used in the industry is 3210×1350 mm.
In the context of the present invention, reflective element will be understood as the assembly formed by the mirror and the underframe on which it is mounted. This underframe consists of a generally metal support structure manufactured from metal sections attached to one another or press-formed sheet metal. Its function is to provide rigidity to the assembly, maintain the curvature in those cases in which the reflective sheet requires it and enable the mechanical attachment between the mirror and the structure of the heliostat.
Generally, each of the reflective elements is provided with a slight curvature in order to be able to direct the solar radiation towards the focus of the field. The degree of curvature will depend on the position of the reflective element in the solar field.
The technical features required of the reflective elements for efficiently performing their work of concentrating solar radiation in a focus are:                Precision curvature in order to be able to concentrate the reflected long-distance radiation.        Reflectance values above 93% to maximize the ratio of the reflected radiation to the incident solar radiation.        Weathering resistance, maintaining the mechanical and physical properties throughout the service life of the solar field.        
In addition to complying with the specifications described above, the reflective elements must contribute to the economic viability of the solar field so it must be possible to manufacture them in mass production conditions using low-cost elements.
The main problems of the current technology are those derived from the design and manufacturing process of the underframe holding the mirrors of the heliostats.
As discussed, these underframes are generally formed by an assembly of metal sections mechanically attached by means of welds or threaded attachments forming a metal support structure with an outer frame that is usually rectangular.
A first problem with this design is the excessive weight of the structure, derived from the nature of metal materials. This high inertia entails an added difficulty for the heliostat to perform the solar tracking movements. This type of underframe further presents a weakness against climatic agents (corrosion due to humidity, expansion due to abrupt temperature changes, etc.) at those points of attachment between the different metal elements, which may result in enormous problems in the long term.
The high weight of these underframes further limits the size of the mirrors making up the reflective sheet of the heliostat, therefore requiring a larger number of mirrors and accordingly a cost increase.
A third and very important problem is the high cost for manufacturing these structures as a result of the high geometric precision required to achieve good interception. This requires working with a high mechanical precision and high dimensional quality, which in methods of manufacture which include welds and mechanical attachments involves high manufacturing costs. This is aggravated in those cases in which the reflective sheet is directly glued to the metal structure with adhesives, because combined with the difficulties of achieving high construction precision in a curved underframe are the irregularities that are typical of extending the adhesive on a thin reflective sheet which, at long distances, involve a substantial loss in interception.
Document ES2351755A1 proposes a solution to the problem of precision curvature by means of using fixing parts which are adhered directly to the surface of the mirrored glass and enable curving the part on the underframe without needing to curve the latter. However, this system also requires long times for manual adjustment and does not solve the problems derived from the metal structure of the underframe.
In document ES2326586A1, the underframe based on metal sections is replaced with an underframe made of lightweight sheet metal that is press-formed by drawing processes, conferring to it less weight and faster execution time. However, the reflective element continues to be made up of two independent units, the underframe and reflective sheet, to be attached to one another. In this case, the attachment is by means of adhering such that the contact between reflective sheet and underframe occurs in different localized areas of the underframe, which requires a very precise machining and is a source of optical problems, both because of the existence of gluing areas in different positions and those derived from the uniformity of the layer of adhesive. All these sources of error are maximized and become, at a long distance, focal deviations separating the reflected ray from the receiving focus.
All the previously described solutions share the common factor that the underframe supports the mirror by its lower face, the latter having its entire side surface exposed. The reflective sheets are generally made up of a 3 mm monolithic glass mirror without any type of treatment to improve its mechanical properties, for example, annealing. In these conditions, the mirrors are a weak point when performing cleaning and maintenance tasks typical of the solar field, being a source of frequent breaks and the subsequent personal safety problem this presents.
In addition to the aforementioned problems, reflectors of this type with a metal underframe described in the paragraphs above present the problem of controlling peripheral corrugations as a result of the forming process for obtaining the desired curvature in the glass, since said forming process is mechanically performed, which, in the long run, translates into a non-uniform reflection at the edges when the target reflection for optimizing the efficiency of a tower thermal solar plant is a perfect as possible circle.
Document ES512606 claims a method for manufacture solving part of the aforementioned problems of excessive weight and of breaks in the field by means of using a sandwich-type reflective element. However, this reflective element does not eliminate the optical problems derived from using adhesives for gluing the glass mirror to a more or less planar substrate, nor does it allow manufacturing reflective elements having a large surface. Furthermore, the method for manufacture is carried out in two lamination steps. In a first step, the thin glass mirror sheet is laminated by means of using a conventional adhesive on a backing sheet and in a second step, this assembly is laminated again on a second backing sheet to form the end composite reflector. This dual step increases the manufacturing times and therefore increases the process costs. The first step of lamination further limits the maximum dimensions of the reflector due to the differences of the expansion coefficients between the glass substrate and the sheet on which it is directly glued.
A critical point in the existing processes of sandwich-type reflectors in the existing manufacturing processes and that the known solutions do not approach with clarity and efficiency is the control of the precision of the optical geometry of concentration because of the differences in the coefficients of expansion of the materials to be attached.
In the context of the present invention optical precision is understood as the statistical measurement obtained as the square root of the root mean square or RMS of the angular deviation values of the reflective surface. In the current state of the art, a spherical reflector valid for performing the functions of concentrating solar energy in a tower-type thermal solar plant must have a surface optical precision value less than 1.5 mrad.